1. Field of the Invention
This invention relates generally to two-cycle engines and more particularly to two-cycle engines with means for varying a port timing in accordance with the engine speed to improve power output.
2. Prior Art
As is well known, in a two-cycle internal combustion engine, an exhaust port is selectively closed by a reciprocating piston in a cylinder during the operation of the engine.
One conventional port timing control device as disclosed in Japanese patent publication No. 47-36047 comprises a port timing control valve comprising an elongated valve element of an arcuate plate and a lever carrying at one end the valve element and pivotally mounted on an upper portion of a wall of the exhaust passage, so that the valve element is movable between an open position where the valve element is received in a recess formed in a lower portion of the wall of the exhaust passage and a closed position where the valve element closes an upper portion of the exhaust port to delay an exhaust timing. This conventional port timing control device has been found not entirely satisfactory, however, in that when the valve is in its open position with the valve element received in the recess, the pivotal lever extends through the exhaust passage diametrically thereof adjacent to the exhaust port and therefore affects the flow of the exhaust gas from the exhaust port.
Another conventional port timing control device, as disclosed in Japanese patent application Laid-Open (Kokai) No. 58-42868 and Japanese Utility Model Publication No. 58-5063, comprises a port timing control valve of a generally semicylindrical shape mounted on a wall of an exhaust passage for angular movement about an axis perpendicular to the axis of the cylinder between an open position where the valve is held in a recess formed in an upper portion of the exhaust passage wall and a closed position where the valve is held in the exhaust passage with a peripheral surface thereof, serving as a timing control surface, closing an upper portion of the exhaust port to delay an exhaust timing. The timing control surface of the valve is held in sliding contact with a complementarily-contoured surface of the recess. Unburned products, such as carbon, contained in an exhaust gas discharged from the combustion chamber via the exhaust port tends to deposit on the timing control surface of the valve when the valve is in its closed position. Therefore, it is quite possible that the deposited products prevent the valve from moving smoothly between its open and closed positions. In addition, the generally semi-cylindrical valve is reduced in cross-sectional area progressively from its opposite ends toward a center thereof, so that it is rather difficult for the peripheral timing control surface to be closely fitted on the peripheral surface of the piston when the valve is in its closed position.